GROUP BY DAY, CUMULATIVE SUM - sql-server

Question

I have a table in MSSQL with the following structure:
PersonId
StartDate
EndDate
I need to be able to show the number of distinct people in the table within a date range or at a given date.
As an example i need to show on a daily basis the totals per day, e.g. if we have 2 entries on the 1st June, 3 on the 2nd June and 1 on the 3rd June the system should show the following result:
1st June: 2
2nd June: 5
3rd June: 6
If however e.g. on of the entries on the 2nd June also has an end date that is 2nd June then the 3rd June result would show just 5.
Would someone be able to assist with this.
Thanks
UPDATE
This is what i have so far which seems to work. Is there a better solution though as my solution only gets me employed figures. I also need unemployed on another column - unemployed would mean either no entry in the table or date not between and no other entry as employed.
CREATE TABLE #Temp(CountTotal int NOT NULL, CountDate datetime NOT NULL);
DECLARE #StartDT DATETIME
SET #StartDT = '2015-01-01 00:00:00'
WHILE #StartDT < '2015-08-31 00:00:00'
BEGIN
INSERT INTO #Temp(CountTotal, CountDate)
SELECT COUNT(DISTINCT PERSON.Id) AS CountTotal, #StartDT AS CountDate FROM PERSON
INNER JOIN DATA_INPUT_CHANGE_LOG ON PERSON.DataInputTypeId = DATA_INPUT_CHANGE_LOG.DataInputTypeId AND PERSON.Id = DATA_INPUT_CHANGE_LOG.DataItemId
LEFT OUTER JOIN PERSON_EMPLOYMENT ON PERSON.Id = PERSON_EMPLOYMENT.PersonId
WHERE PERSON.Id > 0 AND DATA_INPUT_CHANGE_LOG.Hidden = '0' AND DATA_INPUT_CHANGE_LOG.Approved = '1'
AND ((PERSON_EMPLOYMENT.StartDate <= DATEADD(MONTH,1,#StartDT) AND PERSON_EMPLOYMENT.EndDate IS NULL)
OR (#StartDT BETWEEN PERSON_EMPLOYMENT.StartDate AND PERSON_EMPLOYMENT.EndDate) AND PERSON_EMPLOYMENT.EndDate IS NOT NULL)
SET #StartDT = DATEADD(MONTH,1,#StartDT)
END
select * from #Temp
drop TABLE #Temp

Answer 1

You can use the following query. The cte part is to generate a set of serial dates between the start date and end date.
DECLARE #ViewStartDate DATETIME
DECLARE #ViewEndDate DATETIME
SET #ViewStartDate = '2015-01-01 00:00:00.000';
SET #ViewEndDate = '2015-02-25 00:00:00.000';
;WITH Dates([Date])
AS
(
SELECT #ViewStartDate
UNION ALL
SELECT DATEADD(DAY, 1,Date)
FROM Dates
WHERE DATEADD(DAY, 1,Date) <= #ViewEndDate
)
SELECT [Date], COUNT(*)
FROM Dates
LEFT JOIN PersonData ON Dates.Date >= PersonData.StartDate
AND Dates.Date <= PersonData.EndDate
GROUP By [Date]
Replace the PersonData with your table name
If startdate and enddate columns can be null, then you need to add
addditional conditions to the join
It assumes one person has only one record in the same date range

Answer 2

You could do this by creating data where every start date is a +1 event and end date is -1 and then calculate a running total on top of that.
For example if your data is something like this
PersonId StartDate EndDate
1 20150101 20150201
2 20150102 20150115
3 20150101
You first create a data set that looks like this:
EventDate ChangeValue
20150101 +2
20150102 +1
20150115 -1
20150201 -1
And if you use running total, you'll get this:
EventDate Total
2015-01-01 2
2015-01-02 3
2015-01-15 2
2015-02-01 1
You can get it with something like this:
select
p.eventdate,
sum(p.changevalue) over (order by p.eventdate asc) as total
from
(
select startdate as eventdate, sum(1) as changevalue from personnel group by startdate
union all
select enddate, sum(-1) from personnel where enddate is not null group by enddate
) p
order by p.eventdate asc
Having window function with sum() requires SQL Server 2012. If you're using older version, you can check other options for running totals.
My example in SQL Fiddle
If you have dates that don't have any events and you need to show those too, then the best option is probably to create a separate table of dates for the whole range you'll ever need, for example 1.1.2000 - 31.12.2099.
-- Edit --
To get count for a specific day, it's possible use the same logic, but just sum everything up to that day:
declare #eventdate date
set #eventdate = '20150117'
select
sum(p.changevalue)
from
(
select startdate as eventdate, 1 as changevalue from personnel
where startdate <= #eventdate
union all
select enddate, -1 from personnel
where enddate < #eventdate
) p
Hopefully this is ok, can't test since SQL Fiddle seems to be unavailable.